1. Field of the Invention
This invention is concerned with the catalytic cracking of metal-contaminated oils in the absence of added hydrogen. In particular, it is concerned with the fluid catalytic cracking of heavy hydrocarbon oils, such as residual oils, that contain substantial quantities of metal. Partial demetallation of such metal-contaminated oils followed by fluid catalytic cracking is another aspect of this invention.
2. Background of the Invention
Fluid catalytic cracking of hydrocarbon oils is a major refinery process. The installed plants are characteristically large, and are usually designed to process from about 5,000 to 135,000 bbls/day of fresh feed. Briefly, the catalyst section of the plant consists of a cracking section where a heavy chargestock is cracked in contact with fluidized cracking catalyst, and a regenerator section where fluidized catalyst coked in the cracking operation is regenerated by burning with air. All of the plants utilize a relatively large inventory of cracking catalyst which is continuously circulating between the cracking and regenerator sections. The size of this circulating inventory in existing plants is within the range of 50 to 600 tons. Because the catalytic activity of the circulating inventory of catalyst tends to decrease with age, fresh makeup catalyst usually amounting to about one to two percent of the circulating inventory, which corresponds to about 0.1 to 0.25 lbs. per bbl. of fresh feed, is added per day to maintain optimal catalyst activity, with daily withdrawal plus losses of about like amount of aged circulating inventory, commonly referred to as "equilibrium" catalyst.
In general, the oils fed to this process are principally the petroleum distillates commonly known as gas oils, which boil in the temperature range of about 650.degree. F. to 1000.degree. F., supplemented at times by coker gas oil, vacuum tower overhead, etc. These oils generally have an API gravity in the range of about 15 to 45 and are substantially free of metal contaminants.
The chargestock, which term herein is used to refer to the total fresh feed made up of one or more oils, is cracked in the reactor section in a reaction zone maintained at a temperature of about 800.degree. F. to 1200.degree. F., a pressure of about 1 to 5 atmospheres, and with a usual residence time for the oil of from about one to ten seconds with a modern short contact time riser design. The catalyst residence is from about one to fifteen seconds. The cracked products are separated from the coked catalyst and passed to a main distillation tower where separation of gases and recovery of gasoline, fuel oil, and recycle stock is effected.
Petroleum refiners usually pay close attention in the fluid catalytic cracking process (hereinafter referred to as the FCC process) to supplying feedstocks substantially free of metal contaminants. The reason for this is that the metals present in the chargestock are deposited along with the coke on the cracking catalyst. Unlike the coke, however, they are not removed by regeneration and thus they accumulate on the circulating inventory. The metals so deposited act as a catalyst poison and, depending on the concentration of metals on the catalyst, more or less adversely affect the efficiency of the process by decreasing the catalyst activity and increasing the production of coke, hydrogen and dry gas at the expense of gasoline and/or fuel oil. Excessive accumulation of metals can cause serious problems in the usual FCC operation. For example, the amount of gas produced may exceed the capacity of the downstream gas plant, or excessive coke loads may result in regenerator temperatures above the metallurgical limits. In such cases the refiner must resort to reducing the feed rate with attendant economic penalty. Thus, a catalyst inventory that contains excessive deposits of metal is normally regarded as highly undesirable.
The principle metal contaminants in crude petroleum oils are nickel and vanadium, although iron and small amounts of copper also may be present. Additionally, trace amounts of zinc and sodium are sometimes found. It is known that almost all of the nickel and vanadium in crude oils is associated with very large nonvolatile hydrocarbon molecules, such as metal porphyrins and asphaltenes. Crude oils, of course, vary in metal content, but usually this content is substantial. An Arab light whole crude, for example, may assay 3.2 ppm (i.e. parts by weight of metal per million parts of crude) of nickel and 13 ppm of vanadium. A typical Kuwait whole crude, generally considered of average metals content, may assay 6.3 ppm of nickel and 22.5 ppm of vanadium. Regardless of the crude source, however, it is known that distillates produced from the crude are almost free of the metal contaminants which concentrate in the residual oil fractions.
Petroleum engineers concerned with the FCC process have several ways for referring to the metal content of a chargestock. One of these is by reference to a "metals factor", designated F.sub.m. The factor may be expressed in equation form as follows: EQU F.sub.m =ppm Fe+ppm V+10 (ppm Ni+ppm Cu)
A chargestock having a metals factor greater than 2.5 is considered indicative of one which will poison cracking catalyst to a significant degree. This factor takes into account that the adverse effect of nickel is substantially more than that of vanadium and iron present in equal concentrations with the nickel.
Another way of expressing the metals content of a chargestock is as "ppm Nickel Equivalent", which is defined as EQU ppm Nickel Equivalent=ppm nickel+0.25 ppm vanadium
For the purpose of this specification, we shall use the ppm Nickel Equivalent designation in discussing metals content of metal-contaminated oils, distillate stocks, and catalysts. As shown above, no mention is made of copper because this metal usually is not present to any significant extent. However, it is to be understood herein that if it is present in significant concentration, it is to be included in the computation of Nickel Equivalent and weighted as nickel.
It is current practice in FCC technology to control the metals content of the chargestock so that it does not exceed about 0.25 ppm Nickel Equivalent. Catalyst make-up is managed to control the activity of the circulating inventory. With this practice, for example, in a plant utilizing 50,000 bbls/day of fresh feed, and an equilibrium catalyst withdrawal of 9 tons per day, the withdrawn catalyst under steady state conditions will contain about 300 ppm Nickel Equivalent of metals, taking into account that the fresh catalyst contributes 70 ppm to this value. Thus, the circulating inventory is maintained at about 300 ppm Nickel Equivalents of metal, which is considered tolerable, the usual range being at about 200 to 600 ppm, with preferred operating being at about 200 to 400 ppm. It is to be understood, of course, that the metals content of the chargestock may vary from day to day without serious disruption, provided that the weighted average of the metals content does not exceed about 0.25 ppm nickel equivalent of metal.
It is important, for the purpose of the present invention, to understand that all references to the metals content of an oil, or of a chargestock, refer to the time-weighted average taken over a substantial period of time such as one month, for example. Because of the large inventory to catalyst relative to the total metals introduced into the system by the chargestock in one day, for example, the metals content of the catalyst changes little each day with fluctuations in the quality of the chargestock. However, a persistent increase in the metals content of the latter will in time result in a well-defined, calculatable increase in the metals content of the circulating inventory of catalyst, which determines the performance of the FCC unit. In fact, it is evident that the circulating inventory of catalyst, by its metals content, provides a time-average value of the metals content of the chargestock. It is in this context, then, that the phrase "metals content of the chargestock" is used herein.
For the purpose of this invention, chargestocks to the FCC process that contain up to about 0.40 ppm Nickel Equivalent of metal contaminants will be regarded as substantially free of metal contaminants. Chargestocks that contain at least about 0.50 ppm Nickel Equivalents of metal will include those chargestocks referred to as metal-contaminated.
The effects of nickel, vanadium and other heavy metals on activity and selectivity of FCC catalysts are discussed in detail by Cimbalo, Foster and Wachtel in a paper presented at the 37th midyear meeting of the API Division of Refining under the title "Deposited Metals Poison FCC Catalyst" and published at pages 112-122 of the Oil and Gas Journal for May 15, 1972, the full contents of which are incorporated herein by reference. Those authors show metal contaminants of cracking catalyst decline in poisoning activity through repeated cycles of oxidation and reduction and propose a value of "effective metals" determine by multiplication of actual metals concentration by a fraction related to the rate of fresh catalyst make-up as percent of catalyst inventory. Although the authors note that different cracking catalysts may respond differently to metal poisoning and that differences in operation of the regenerator may affect rate of metal deactivation, they establish a single standard for determination of "effective metal" values to be applied generally, presumably having regard to specific catalyst and operating conditions.
In addition to the heavy metals discussed above, residual stocks may contain alkali metals, primarily as salts such as sodium chloride derived, for the most part, from brines which are found in association with crude petroleum in the formations from which the petroleum is produced. Such salts, e.g. sodium chloride, are troublesome in refinery equipment generally, tending to deposit on fractionating trays or packing in distillation columns and elsewhere. It is conventional to "desalt" crudes which contain substantial quantities of this inorganic impurity by washing with water and settling, together with breaking of stubborn emulsions which often are present in the crude or formed during water washing. Such salt as may remain in the crude will, in due course of distillation, be found in the residual fraction.
The residual fraction of single stage atmospheric distillation or two stage atmospheric/vacuum distillation also contains the bulk of the crude components which deposit as resinous or tar-like bodies on cracking catalysts without substantial conversion. These are frequently referred to as "Conradson Carbon" from the analytical technique of determining their concentration in petroleum fractions. The Cimbalo article above cited classifies coke on spent catalyst in four groups: catalytic coke resulting from cracking of charge components; cat-to-oil, related to reactor stripper efficiency; carbon residue (Conradson) as just discussed; and contaminant coke derived from dehydrogenation reactions promoted by the heavy metal poisons nickel, vanadium, etc. The residual stocks not only provide metal poisoning of the catalyst but also show high Conradson Carbon values which are reflected by coke of that class very nearly equal to the Conradson Carbon number. It will be seen that the increment of Conradson Coke results from deposition on the catalyst of non-volatile hydrocarbons in the charge without significant change in nature of the deposited hydrocarbons.
With very limited exceptions, residual oils have not been successfully included in the chargestocks to the FCC process. The reaons for this are not fully understood, although from the foregoing discussion it is apparent that their high metals content is certainly a major contributing factor, as is the typically high Conradson Carbon. There has been interest in using them, however. The reason for this interest becomes apparent when we consider, for example, that typically only about 26 volume % of the Arab light whole crude is the 650.degree.-1000.degree. F. gas oil fraction, while the total 650.degree. F. plus resid constitutes about 43 volume %. Thus, were it feasible to efficiently operate with residual oil fractions, a very substantial increase in the amount of gasoline plus fuel oil derivable from a barrel of crude could be obtained. In some refineries, the vacuum resid remaining after the distillation of the gas oil is coked and the coker gas oil is included in the FCC chargestock. However, it is generally recognized that coker gas oil, because of its high unsaturated and high aromatics content, is a poor quality feed.
It has been proposed in the prior art to hydrotreat residual oils under such conditions that the metals content is brought into the range commonly associated with gas oils. Such hydrotreated residual oils, substantially free of metal contaminants, may then be used as chargestock or a component thereof fo the FCC process. Proccesses to achieve such metals and sulfur reduction are disclosed in U.S. Pat. No. 3,891,541, issued June 24, 1975 and U.S. Pat. No. 3,876,523, issued Apr. 8, 1975, for example, the entire contents of which are incorporated herein by reference. The combination of hydrotreating to reduce metals and sulfur content followed by cracking also is disclosed in a publication by Hildebrand et al. in The Oil and Gas Journal, pp 112-124, Dec. 10, 1973, the entire contents of this article being incorporated herein by reference. However, no installation is known which has adopted the proposed scheme, probably because the cost and severity associated with the operation involves a heavy economic penalty.
It is one object of this invention to provide an improved process for the fluid catalytic cracking of metal-contaminated hydrocarbon oils. It is a further object of this invention to provide a method for the fluid catalytic cracking of residual petroleum oils which is highly selective for the production of liquids in the motor fuel and heating oil boiling ranges. These and other objects of this invention will become evident to those skilled in the art from reading this entire specification including the claims thereof.